Odd Audio Issue With Video Conference

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We’re experimenting with using Asterisk (14.6.0) for video conferences. This test has three endpoints, a Polycom Trio with its video accessory, and two desktops running Linphone. The video is all H.264. We’re using Opus for audio on the Linphone Windows desktops and have tried both G.722 and Siren14 on the Polycom.

When we have a two-party conference, everything works fine. But when we add a third party, it gets odd. The two Linphone users can hear each other just fine and the Polycom user can hear the Linphone users fine, but when the Polycom user starts talking, all is OK for about four seconds, then it gets replaced by a hiss for the rest of the period of talking, then goes back to being OK and repeats.

This is peculiar. It doesn’t sound like an Asterisk bridge issue because it only happens for one particular participant, but it’s hard to see how it can be a Polycom issue since it can’t tell between two and three participants.

Does anybody have any ideas here about what to try next to see if we can diagnose this?